. Comparative anatomy and physiology. 52 COMPARATITE ANATOMY AND PHYSIOLOGY. delicate branches with terminal orifices ; the two vessels open into a special enlargement or bladder, the walls of which are contractile, so that the fluid stored up in it can be forced to the exterior. The sexes, as in JSTematoids, are ordinarily separate, and the males can be distinguished from the females by their smaller size. Rotifer, Brachionus, and Melicerta are examples of the Rotatoria. Most of the forms with which we have already had to do are small in size, and it will have been noted that, where the body
. Comparative anatomy and physiology. 52 COMPARATITE ANATOMY AND PHYSIOLOGY. delicate branches with terminal orifices ; the two vessels open into a special enlargement or bladder, the walls of which are contractile, so that the fluid stored up in it can be forced to the exterior. The sexes, as in JSTematoids, are ordinarily separate, and the males can be distinguished from the females by their smaller size. Rotifer, Brachionus, and Melicerta are examples of the Rotatoria. Most of the forms with which we have already had to do are small in size, and it will have been noted that, where the body attained, as in the case of certain tape-worms, to a con- siderable length, that body was not an individual whole, but was broken up into joints or segments. In the great group of worms which we are now going to consider, this segmentation of the body is very distinctly exhibited, and affects not only the external form, but the great majority of the internal organs ; this phenomenon becomes the more comprehensible when we -, ,-, * • , T learn that at one of its very earliest stages in development the mesoblast itself becomes regularly segmented. In the simpler conditions the segments, which we will henceforward call metaineres, are, for the greater part, exactly similar in character, and only those at either end of the body differ much from the rest. Later on we shall see that, just as in the simpler animals, different parts take on different duties, and division of labour becomes as apparent among the metameres as it was in the various persons of the colonial Coelenterata. % Now, also, we find that organs for which, in the smaller and simpler forms, there was no necessity,. Fig. 19. to show the Ciliated Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Bell, F. J. (Francis Jeffrey), 1855-1924. L
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